This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Traditionally, it is understood that centrifugal compressors or gas expanders do not handle liquid slugs and thus it is assumed that they can only handle a fraction of one percent liquid by volume. Thus in many applications expensive liquid separators, dehydration processes and/or unit scrubbers are utilized to try and remove or separate the liquids prior to using centrifugal compressors or expanders. These devices are often designed for specific operating conditions and are then limited in the range of Gas Volume Fraction (GVF) that can be handled with a given process flow rate. Even with this expensive and complex processing equipment, if there is a sudden high level of liquids they can quickly saturate, fill and overflow the liquid separators once their capacity for liquid is exceeded resulting in slugging the compressor or expander equipment.
In general, multiphase pumps can be used if it is known that the fluid will generally be below 90% GVF. Centrifugal compressors are often restricted to applications with GVFs of 99.7 or higher and even this can cause problems within the machine for stability and affecting the reliability of the seals and bearings. Therefore, for processes outside this small range, the current practice is to separate the fluids prior to utilizing a centrifugal compressor even with the design limitation with the associated process and equipment. The same is true for gas expanders, which are functionally a centrifugal compressor running in reverse to extract energy in one form or another through a process pressure drop across the expander. The separators, scrubbers and dehydration units are not only expensive and limited in liquid capacity and volume flow range but they also tend to be very bulky, taking up expensive real estate in locations such as offshore platforms, subsea processing or onshore facilities. This coupled with complex control systems and additional auxiliary equipment like pumps, regulators, level controllers, transmitters and filters adds to the complexity and likelihood of failure of these systems. An example of a typical oil or gas well stream service process may use a separator to separate liquids from the gas in order to prevent or mitigate damage caused by slugs. A centrifugal compressor and pump may subsequently be used to boost the gas and liquid separately, with downstream recombination of the gas and liquid in order to transport both through a pipeline to a processing facility.
Problems with compressing liquids include reduced machine stability, erosion of impellers and diffusers, and fouling and resulting in imbalance if the liquids flash or vaporize while being compressed in the machine.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. Technology that would improve the ability of compressors or expanders to handle the multiphase flow of fluid with a higher liquid content compared to the current state of the art would be of great value.